This study is divided into two parts, as follows: I. Investigation of the pathology of selected viral infections involving the nervous system in experimental animals with the aid of an electron microscope. The study is intended to provide new information on the type and range of pathological changes seen in viral infections with particular emphasis on teratogenic effects, effects on myelin and on the changes seen in subacute and chronic infections. II. The study of human disease of suspected viral etiology will be based on post mortem material and on material obtained at the time of diagnostic cerebral biopsy. This portion of the study will be aimed at the identification of the etiologic agent and at further delineation of pathologic changes indicative of viral infection. Through the use of a combined clinical and experimental pathological approach, it should be possible to advance our understanding of slow virus infections and of teratogenic effects of viruses.